Process for converting mineral oils



Oct. 1. 1940- A. P. FRAME ET AL ,4

PROCESS FOR CONVERTING MINERAL OILS Filed Dec. 29, 1936 Q. IN.

PRESSURE Pouuos I0203040506070809OIOOIIOI20|3OI4OI5O Z5 STLL TUBES AIRgs-Am 34 on. m :2 X k 3, 46 2 I 36 INVENTORS 1, u'xnac'xnynnxx,,]///////;/,6963261114 ALFRED R FRAME I WILHAM H. PRICE 4 2g 35 ATTbRNEYPatented Oct. 1, 1940 PATENT OFFICE rnoonss FUR CONVERTING MINERAL OILSAlfred Putnam Frame, Haddonfield, and William Harold Price, Camden, N. Jassignors to Powerv Patents Company, Hillside, N. J a corporation ofMaine Application December 29, 1936, Serial'No. 118,040

10 Claims.

This invention relates to an improved process and apparatus for crackingmineral oils and more particularly to a controlled method of effectingsuch crackingin pipe still heaters.

The present practice employed in the cracking of mineral oils,regardless of the type of process, almost invariably involves the useofa pipe still heater or furnace in which-the oil to be cracked ispassed through two ormore banks of tubes mounted in the furnace.- Someof these banks are subjected to radiant heat from the combustion takingplace in the furnace fire box, while most furnaces include at least oneconvection bank of tubes shielded from direct radiation. It is quiteusual for certain of the tubes of this pipe still furnace to becomecoated inside with a thin layer of carbon or coke, particularly wherethe furnaces are used for cracking oils at high inlet pressures offrom400 to 800 pounds per square inch, and at transfer line temperaturesof from 890 to 950 F. Experiments-have shown that this coating ofcarbon'or cokeon the inside walls of certain tubes occurs in aparticular part of the furnace and that after a certain period of timethe coatingbecomes so important that the cracking unit must be shutdownand the tubes cleaned. y

The investigation of this coke" formation disclosed the fact that forexample, in the cracking of topped crude oil at furnace'outlettemperatures of about 915'-F.-the coking in the furnace tubes occurredat -a point where theoilite'mperature was between about 860 and 890 F.,when operating the furnace at an inlet'pressure of about 575 lbs. persquare inch and an outlet pressure of approximately 250 lbs. per'squareinch; Having in mind the-foregoing difficulties and disadvantagesresulting from the usual pipe still furnace practice, the primary objectof the present invention is to provide a process which will overcomethese difliculties and at the same time secure as good or better resultsfrom the heating and cracking operation. I

A further object of the invention is the provision of a process forcracking mineral oils in which the pressure, temperature and heatingconditions to which the oil is subjected in the pipe still furnaceduringthe cracking operation are correlated-in such a way as to overcomethe disadvantages referred to. Another object of the invention is theprovision of the improved apparatus for carrying out the process of thepresent invention.

According to the improved process, the present invention comprises thesteps, of passing theoil to be cracked through two or more tube banksmounted in a pipe still heater in which the temperature and pressureconditions imposed upon the oil are correlated in such a way that cokeis not deposited-as in prior operations. The process also includes thestep of controlling the temperature and pressure at various points inthe stream of oil through the pipe still heater in relation to thetemperature to which the oil is heated as it advances through theheater.- For example, the operation is controlled to maintain apressurebelow approximately 400 lbs. per square inch on the oil after it-reachesa temperature of approximately 860 F. The process ofthe presentinvention is particularly adaptedfor the cracking of stocks containingresidual oil.

The improved apparatus for carrying'out the process of the presentinvention comprises a'pipe still heater provided with a combustion chamber and a convection heating chamber separated by a bafiie wall aroundone side of'which products of combustion flow from the combustionchamber into the convection chamber. A plurality of similar tubes forheating mineral oil are mounted in the pipe still heater in a positionto receive radiant heat from the combustion chamher, and a bank of tubessimilar'to those receiving radiant heat is mounted in the convectionchamber and heated by combustion gases from the combustion chamber. Thetubes'of the bank in the convectionchamber are connected to form atleast two separate coils of similar length for the" passage of oiltherethrough. Means are provided for introducing oilinto, -and forpassing it in series, through the tubes receiving radiant heat from thecombustion chamber. The apparatus includes means for passing the oilproducts leaving said tubes, and for uniformly distributing the productsto the coils of the tube bank mounted in the convection chamber. Meansare also provided for conducting the heated oil products from the coilsof the convection bank into a transfer line for the pipe still heater.

Other objects and advantages of the process of the present inventionwill be more readily understood from the following detailed descriptiontaken in connection with the accompanying drawing, in which:

. Fig. 1 is a chart which shows by means of I curves the pressures atvarious points in the pipe still furnace under the prior practice andunder the new practice employed in accordance with the features of thepresent invention.

Fig. 2 is a diagrammatic vertical sectional view of .a pipe stillfurnace showing the relationship of tube banks and circuit suitable forcarrying out the features of the present invention.

Referring to Fig. 2 of the drawing, the improved apparatus employed forcarrying out the process of the present invention may be brieflydescribed as follows: The oil to be cracked which may be fresh chargingstock or a mixture of preheated charging stock and reflux condensate ata temperature of approximately 650 F. is passed under high pressurethrough a furnace inlet line 2 and then through an economizer tube bank4 mounted in the flue gas outlet from the furnace. The oil then passesthrough a connecting line 6 and through a bank of radiant heating tubes8 mounted vertically along the wall of the combustion chamber of thefurnace. From the tube bank 8 the heated oil passes through a connectingline l0 and a radiant roof bank l2. A connecting line l4 from the bankl2 conducts the heated oil to the lower portion of the furnace where itis divided into two streams and passed through a convection bank oftubes IS in parallel streams. The products are discharged from this bankof tubes through lines 20 which lead to a transfer line 22 provided witha valve 24.

The banks of tubes in the pipe still furnace are heated by hot productsof combustion supplied fr'om a plurality of burners 26 through paralleltunnels 28 in the bottom of the furnace as shown. The products from thetunnels 28 discharge into a combustion or fire box chamber 30 to theleft of which is mounted the radiant tube bank 8. The tube bank l2mounted above the combustion chamber also receives radiant heat. Theproducts of combustion from the furnace pass upwardly through thecombustion chamber and over a thin heat conducting bridge wall 32, andthen downwardly over the tubes of the convection bank [8 and theeconomizer bank 4. The spent products of combustion are discharged fromthe furnace through a conduit 34 and a portion of these hot products arerecirculated back to the combustion chamber by means of a connectingbranch conduit 36, a blower 38 and a conduit 40. The recycled productsof combustion are discharged into the combustion chamber at pointsopposite the outlet of tunnels 28.

; The flue gas which is not recirculated through the furnace passes toan air preheater 42 from the pipe 34 and finally to a stack from anoutlet conduit 44. The air for use in the furnace is passed through aninlet pipe 46 and in heat exchange with the flue gas in preheater 42.The preheated air is conducted to the burners 26 from a conduit 48.

. The process of the present invention relates primarily to thetreatment of the oil in the pipe still furnace, but it is to beunderstood that the cracked products discharged throughthe transfer line22 may be subjected to a time reaction by introducing them, for example,into the upper part of an enlarged vertical reaction chamber (shownmostly broken away). The materials from the reaction chamber arefractionated to remove the residual constituents and separate thedistillate products into gasoline and higher boiling fractions.

In a typical prior operation in which a furnace of the type shown inFig. 2 was employed, but where the convection tube bank l8 was a singleseries tube bank, the oil was charged through the line 2 at a pressureof approximately 575 pounds per square inch in order to obtain anoutletlpressure in the transfer line 22 of approximately'250 lbs. persquare inch. In this operation, the charging stock was a mixture oftopped crude oil and gas oil recycle stock produced in the crackingoperation. ,In this process in which the oil temperature in the transferline 22 was about 917 F. the operation was continued for approximately100 hours when it was observed that although the outlet pressure in thefurnace was maintained substantially constant, the inlet pressure hadincreased to such an extent that it was necessary to shut down thecracking unit and clean the tubes. After the unit had been shut down andcooled and the tubes opened, it was found that there was coking only inabout 25 or tubes in the lower part of the convection tube bank. Thecoking in this part of the furnace could not be readily accounted forbecause it was practically in the zone of lowest flue gas and furnacetemperatures. Furthermore it was in the zone in which there was noradiant heating of the tubes.

In a subsequent similar run, the temperatures at various points in thetube banks were determined (after the unit was on stream and stabilized)and it was found that the tubes which had previously been coked were inthe zone in which the oil temperatures ranged from approximately 860 F.to 890 F. In these runs the pressure conditions prevailing in the coilat various points were measured and when plotted, gave the upper curveshown in Fig. 1 of the drawing.

In carrying out the improved process involving the use of the samenumber of tubes in the convection bank 18, but divided into two separatestreams of equal length, it was found that a very much lower pumppressure was required to maintain an outlet pressure of approximately250 lbs. per square inch. The pressure conditions for the new operationare shown in the lower curve in Fig. l, in which the gauge pressure inpounds per square inch is plotted against the number of tubes in thepipe still furnace,

A typical run in accordance with the process of the present invention ona charging stock substantially the same as the mixture referred to abovecontinued for a period of approximately 400 hours when the unit was shutdown for inspection. The tubes in which the coating of coke formerlyoccurred were free of cokeand the remainder of the furnace tubes werealso found to be free from coke. In this operation the furnace inletpressure was approximately 425 lbs. per square inch or approximately 150lbs. per square inch less than the pressure required in the old type ofoperation as shown by the curves in Fig. 1. At this inlet pressure itwas found'possible to increase the feed rate over the old operation byapproximately 30%, and at the same time secure a higher yield ofgasoline which had an equivalent octane number to the, gasoline producedin the old operation.

It appears that the relationships of the particular temperatures andpressures employed in the heating operation contribute to the importantimprovements obtained by the process of the present invention. Thevelocity of the products passing through the various portions of thecoil comprising the tube banks may also affect the success of theoperation. In any case these temperatures have been correlated in such away as to provide an extremely efficient process for the cracking ofhydrocarbon oils in pipe still heaters.

The temperature of 860 to 890 appears to be a critical temperature for amixture of topped crude oil. and gas oil reflux condensate. The newconditions provided for this temperature range have avoided theformationof coke incrustations in the coil. The critical temperature forother stocks containing more, or less residue, may be somewhat higher orlower than this range, but it is possible to control the temperature atvarious parts of the furnace so that any cracking stock may beeffectively treated without the formation of coke in the coil.

From the foregoing description it will be readily apparent to thoseskilled in the art that various other modifications may be made in theprocess and apparatus without departing from the spirit and scope of thepresent invention and such changes are contemplated within the scope ofthe appended claims.

Having thus described the invention in its preferred form, what isclaimed as new is:

1. In the cracking of mineral oils in which the oil to be .cracked ispassed through a plurality of tube banks in a pipe still heater, theimprovement which comprises passing an oil mixture including gas oilrecycle stock at a high initial pressure in excess of 400 lbs. persquare inch in a single stream through the tube banks except the last,and then in a plurality of streams through the last of said tube banks,heating said single stream of oil to a temperature slightly below thatat which the oil reaches a critical state at which coke would bedeposited on the walls of the tubes, heating the plurality of streams ofoil through a temperature range from slightly below said critical stagetemperature to a discharge temperature of from 890 F. to about 950 F.,maintaining a pressure below 400 lbs.

per square inch on the oil of said streams to prevent coke formation inthe tubes through which said streams pass, and maintaining a substantialsuperatmoshperic pressure not exceeding 400 lbs. per square inch on theoil products of said streams as discharged from the last of said tubebanks.

2. The process of cracking mineral oils as defined by claim 1 in whichthe oil feed rate to the pipe still heater is controlled in relation tothe discharge pressure from the heater to maintain a relatively low rateof pressure drop in the plurality of streams of oil flowing through thelast tube bank.

3. In the cracking of mineral oils containing recycle stock in which theoil to be cracked is passed through a plurality of tube banks in a pipestill heater, the improvement which comprises passing the oil to becracked at a high initial pressure substantially in excess of 400 lbs.per square inch in a single stream through the tube banks except thelast and then in a plurality of streams of similar diameter through thelast of said tube banks, heating the stream of oil in the tube banksexcept the last to a temperature slightly below that at which the oilwould deposit coke on the walls of the tubes, heating the plurality ofoil streams in the last of said tube banks through a temperature rangefrom slightly below said coke deposition temperature to a dischargetemperature above 890 F. and not substantially above 950 F., controllingthe operation to maintain a pressure at below approximately 400 lbs. persquare inch on the oil after it reaches a temperature of approximately860 F., and discharging the resulting oil products from said heater at asubstantial superatmospheric pressure.

4. The process of cracking oil as defined by claim 3 in which saidplurality of streams of oil are heated while passing through aconvection through a series of heating zones in a pipe still furnace,and in which under usual operating conditions .coke is deposited in thetubes of the pipe still furnace at a point therein at which the oilpassing therethrough reaches a critical stage at a relatively highpressure, the improvement in such cracking operations for the preventionof such coke formation, which comprises passing an oil mixtureincludingcrude oil residual constituents in a flowing stream into saidpipe still furnace at a relatively high inlet pres-' sure substantiallyin excess of 400 lbs. per square inch, passing the oil of said streamthrough the heating zones of said pipe still furnace and heating the oilas it passes therethrough to a discharge temperature of from 890 F. to atemperature not substantiallyl exceeding 950 F., controlling the heatingoperation in said pipe still furnace to heat the oil of said stream upto a temperature below but approaching that at which the oil woulddeposit coke on the walls of the pipe still tubes, dividing the oilproducts flowing through said heating zones into a plurality of streamsat a point therein immediately preceding that at which the oil wouldreach a critical stage and deposit coke on the walls of the pipestilltubes, whereby coke deposition in the tubes of said pipe stillfurnace is prevented, and discharging the cracked oil products from saidfurnace at a substantial superatmospheric pressure."

7. In the cracking of mineral oils containing recycle stock in which theoil to be cracked is passed under pressure through zones in a pipe stillheater, the improvement which comprises passing the oil to be cracked ina flowing stream into said pipe still heater at a high inlet pressuresubstantially in excess of 400 lbs. per square inch, passing the oil ofsaid stream through said pipe still heater and heating the oilconstituents passing therethrough to a discharge temperature of from 890F. to a temperature not substantially above 950 F., heating the oil ofsaid stream in said heater to a temperature below but approaching thatat which coke would normally be deposited on the walls of the tubes ofthe pipe still and then dividing the oil of said stream into a pluralityof streams at a point preceding that at which said cokedeposition wouldoccur thereby preventing coke deposition in the pipe still tubes,controlling the pressure maintained on the oil flowing through saidheater so that the pressure on the oil is below 400 lbs. per square inchafter the oil of said stream has been divided into a plurality ofstreams, and. discharging the oil products of said heating from saidpipe still heater at a substantial superatmospheric pressure but below apressure of 400 lbs. per square inch.

8. In the process of converting and cracking mineral oils containingrecycle stock in which the oil to be converted is passed through aseries of heating zones in streams of restricted cross section, theimprovement which comprises passing the oil to be cracked and convertedat an initial high pressure substantially in excess of 400 lbs. persquare inch in a. flowing stream of restricted cross section into thefirst of the series of heating zones, passing the oil of said streamthrough the series of heating zones and heating the oil products passingtherethrough to successively raise the temperature thereof and heatingthem to a discharge temperature of from 890 F. to a temperature notsubstantially above 950 F., heating the oil products of said streamintroduced into said series of zones to a temperature below butapproaching that at which coke would be deposited from the oil productsbeing heated and then dividing the Oil products of said stream into aplurality of streams of restricted cross section at a point precedingthat at which coke deposition would occur thereby preventing the depositof coke from the oil streams in said heating zones, and discharging theresulting cracked oil products from the series of heating zones at asubstantial superatmospheric pressure.

9. In the cracking of mineral oils in which the oil to be cracked ispassed through a plurality of tube banks in a pipe still furnace and inwhich the last bank of tubes is a convection tube bank heated by hotproducts of combustion, the improvement which comprises passing the oilto be cracked at a high initial pressure substantially in excess of 400lbs. per square inch in a single stream through the tube banks of saidfurnace except the last, and then in a plurality of streams through thelast of said tube banks, heating the stream of oil in said tube banksexcept the last to a temperature below but approaching that at which theoil would deposit coke on the walls of the tubes, heating the pluralityof oil streams in the last of said tube banks by passing products ofcombustion in a generally countercurrent direction to the flow of saidstreams through said last bank, the oil passing through said last bankbeing heated to a discharge temperature above 890 F., and notsubstantially above 950 F., controlling the opera-.

tion to maintain a pressure below approximately 400 lbs. per square inchon the oil after it reaches a temperature of approximately 860 F., anddischarging the resulting heated oil products from said pipe stillfurnace at a substantial superatmospheric pressure.

10. In the cracking of mineral oils in which the oil to be cracked ispassed under pressure through a series of heating zones in a pipe stillfurnace and in which under usual operating conditions coke is depositedin the tubes of the pipe still furnace when the oil passing therethroughreaches a critical stage temperature at a relatively high pressure, theimprovement in such cracking operations for the preventing of such cokeformation, which comprises passing the oil to be cracked in a flowingstream into said pipe still furnace at a high inlet pressuresubstantially in excess of 400 lbs. per square inch, passing the oil ofsaid stream through the heating zones of said pipe still furnace andheating the oil constituents passing therethrough to a dischargetemperature of from 890 F. to a temperature not substantially above 950F., controlling the heating operation in said pipe still furnace so thatthe pressure on the oil products is below 400 lbs. per square inch afterthe oil products flowing therethrough reach a temperature approachingabout 860 F., providing the oil products flowing through said heatingzones into a plurality of streams at a point in said furnace immediatelypreceding that at which the oil would reach a critical stagetemperature, thereby preventing coke formation in said pipe stillfurnace which would normally occur if the oil products flowing throughthe furnace were not divided into a plurality of streams.

ALFRED PUTNAM FRAME. WILLIAM HAROLD PRICE.

